In recent years, electric storage devices including cells such as a nonaqueous electrolyte cell and capacitors such as an electric double layer capacitor have been greatly put to a variety of uses. In particular, nonaqueous electrolyte cells typified by a lithium-ion battery cell have been frequently used for consumer appliances as, for example, power sources for electronic equipment such as a personal computer developed to be downsized and lightened and communication equipment including a cellular phone and the like, and as power sources or the like for vehicles including a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) and an electric vehicle (EV).
In such an electric storage device, an electrode assembly in which a positive electrode and a negative electrode stacked with a separator sandwiched therebetween are spirally wound is widely used. A positive electrode is formed as an electrode plate having a mixture agent layer formed on a current collector, which is fabricated, for example, by applying a mixture agent in the form of a paste containing a positive electrode active material, a binder and the like onto one face or both faces of a current collector, drying the applied mixture agent and pressing the resulting current collector. Furthermore, a negative electrode is similarly formed by using a negative electrode active material instead of the positive electrode active material.
A current collector is generally made of a rectangular metal foil or the like. In one side portion of the current collector, a non-mixture agent layer part having no mixture agent layer formed therein is provided. When the current collector is spirally wound for fabricating the electrode assembly, the non-mixture agent layer part is disposed on either side along an axial direction of the winding axis, so as to be connected to a lead line.
The mixture agent layer is formed by applying the mixture agent in the form of a paste. Therefore, the paste-form mixture agent sags and its thickness is reduced in an edge portion of the mixture agent layer on the side of the non-mixture agent layer part. If the mixture agent layer is wholly pressed while having such a smaller thickness portion, the density of the mixture agent layer becomes lower in the edge portion of the mixture agent layer on the side of the non-mixture agent layer part than in the other portion. Therefore, adhesiveness of the mixture agent layer becomes poor. If there is such a portion having poor adhesiveness in the edge portion of the mixture agent layer on the side of the non-mixture agent layer part, the mixture agent layer may be peeled off from the current collector during a step of preparing the electrode assembly or during an assembling step of incorporating the electrode assembly into an electric storage device. Thus, there is a problem in which the mixture agent layer is liable to come off from the current collector. In addition, if the mixture agent layer comes off from the current collector, it is apprehended that a part of the mixture agent layer may become a conductive foreign matter within the electric storage device and cause a short-circuit.
Attempts to prevent the mixture agent layer from coming off have been conventionally made. For example, Patent Literature 1 describes a method in which a mixture agent in the form of a paste is applied in a larger thickness in an edge portion for starting application than in the other applied portion in applying the mixture agent onto a current collector, and a resulting mixture agent layer is pressed as a whole after pressing and flattening the edge portion for starting the application. Patent Literature 1 describes that the density in the edge portion for starting the application of the mixture agent is higher than in the other applied portion, and hence the strength of the edge portion can be improved.
Patent Literature 2 describes that the thickness of an edge portion for starting application of a mixture agent in the form of a paste is adjusted to have a thickness ratio of 0.97 to 1.03 to the other applied portion, so that the edge portion for starting the application can be restrained from extremely rising, and hence, the mixture agent can be restrained from coming off at the edge portion.
However, in the method described in Patent Literature 1, the density of the mixture agent layer attained after pressing is set to a prescribed density by adjusting the thickness of the mixture agent layer obtained at the time of application. Therefore, it is necessary to adjust the application thickness of the mixture agent in consideration of thickness reduction caused through drying and pressing. Accordingly, it is difficult to accurately attain a desired density of the mixture agent layer by the method described in Patent Literature 1.
Furthermore, if there arises an excessively large difference in the density between the edge portion of the mixture agent layer and the portion other than the edge portion, a partial difference in hardness may be caused in the mixture agent layer. This may lead to a fear of rupture of the current collector from a portion having higher hardness in winding the current collector for fabricating the electrode assembly. Accordingly, it is necessary to accurately adjust the density difference in the edge portion of the mixture agent layer to an extent in which the adhesive strength of the mixture agent layer can be improved and the current collector is not ruptured. Particularly in an electrode assembly for high power used in a lithium-ion battery cell for an HEV or the like, the mixture agent layer is formed in a small thickness for obtaining high power. Therefore, it is difficult to accurately provide a slight density difference in such a thin mixture agent layer by the aforementioned method.
Also in the method described in Patent Literature 2, it is necessary to set the application thickness at the time of application on the assumption of a thickness attained after drying. Therefore, there arises a problem in which it is difficult to accurately attain a desired thickness of the mixture agent layer. Furthermore, there arises a problem in which it is difficult to provide a slight thickness difference between the edge portion of the mixture agent layer and the portion other than the edge portion.